The present invention relates to apparatus for translating simultaneously occurring, correlated input signals from one frequency or mode to another. More particularly, this invention relates to improvements in apparatus for compensating for so-called "dropouts" (i.e. temporary loss of one or more input signals) in such signal translation apparatus.
The invention is especially suitable for use in a signal translation apparatus which processes recorded signals having carrier components, and particularly multichannel signals which are stereophonically related.
Dropouts have long been a problem in the reproduction of signals from magnetic records such as magnetic tapes, discs and the like. The loss, due to dropouts, of music for more than a normal pause interval, say 15 milliseconds, or of speech for more than the normal intersyllabic interval, is disturbing to the listener. The problem is most severe when the magnetic record is played back at very slow speeds. The dropout then affects a longer portion of the signal than is the case where the recording is made and reproduced at higher speeds. In an FM system, dropouts having a duration of several carrier cycles will produce bursts of noise each time they occur. In audio systems these dropouts will be manifested by annoying "clicks". Thus, any apparatus for compensating for such dropouts must be responsive to the loss of only a few carrier cycles.
Dropouts and loss of signal during transmission and during the recording and reproducing thereof has received attention in the following U.S. Pat. Nos.: 2,996,576 issued Aug. 15, 1961; 4,076,969 issued Feb. 28, 1978; and Reissue 24, 956 issued Mar. 28, 1961. The dropout compensation scheme proposed in U.S. Pat. No. 2,996,576 involves delaying the signal and substituting the delayed signal for the undelayed signal when a dropout is detected. In the apparatus described in U.S. Pat. No. 4,076,969, the delayed signal to be substituted for the dropout is generated in an inertia circuit, specifically a phase locked loop. In the technique described in U.S. Pat. No. Re. 24,956, special signals are recorded which are less prone to the adverse affect of dropouts, and these special signals are used to compensate the information signals for the effects of the dropouts. Various other forms of dropout and lost signal detectors and compensating techniques similar to those mentioned have also been discussed in the patent literature. See, for instance, the disclosures of U.S. Pat. Nos. 3,328,521; 2,937,368; 3,005,189; 3,071,723; 3,588,705; 3,843,573; 4,035,730; and 4,143,325. The main limitation of these techniques is that they do not take full advantage of the information in the transmission in order to provide compensation for dropouts. Accordingly, compensation is not carried out in real time and with the rapidity necessary to prevent dropouts from being subjectively disturbing to the listener or observer.